The U.S. Pharmacopeia XXII, Official Monograph, pp. 1625-1626 begins by defining a biological indicator as "a characterized preparation of specific microorganisms resistant to a particular sterilization process. It is used to assist in the qualification of the physical operation of sterilization apparatus in the development and establishment of a validated sterilization process for a particular article, and the sterilization of equipment, materials, and packaging components for aseptic processing. It may also be used to monitor a sterilization cycle, once established, and periodically in the program to revalidate previously established and documented sterilization cycles. It is in one of two main forms, each of which incorporates a viable culture of a known species of microorganism. In one the spores are added to a carrier (disk or strip of filter paper, glass, or plastic) and packaged so as to maintain the integrity of the inoculated carrier but, when used appropriately in the individual immediate package, so as to allow the sterilizing agent to exert its effect. In the other, the spores are added to representative units of the lot to be sterilized (inoculated product) or to similar units (inoculated similar product)."
A bit later, the Pharmacopeia, at p. 1626, states: "A particular strain of microbial spores selected for use as a biological indicator and resistant to one sterilization process may not necessarily be suitable for other sterilization processes or even differing sterilizing conditions of the same mode of sterilization."
State and federal legislation are severely restricting the amount of hazardous gases, such as ethylene oxide (a carcinogen), in working environments for the use of systems or methods that produce toxic residues or exhaust products. Ethylene oxide has been widely used in hospitals and other areas of the health industry for sterilization processes.
The use of plasma to sterilize containers was suggested in U.S. Pat. No. 3,383,163. Plasma is an ionized body of gas which may be generated by the application of an electromagnetic field to gas. The ionized gas can contact microorganisms on the surfaces of the items to be sterilized and effectively destroy the microorganisms.
Two recently issued patents, of common assignment herewith, use oxidizing gases for sterilization. U.S. Pat. No. 5,084,239, issued Jan. 28, 1992, inventors Moulton et al., describes a process in which an antimicrobial agent treatment is alternated with a downstream plasma treatment. The antimicrobial agent may be peracetic acid vapor under reduced pressure conditions. U.S. Pat. No. 5,115,166, issued May 19, 1992, inventors Campbell et al., describes a plasma sterilizing apparatus and method where an article to be sterilized is exposed to active species made from a plasma. The plasma is generated from a mixture of oxygen; argon, helium, and/or nitrogen; and hydrogen, or with a mixture of air and hydrogen, supplemented by oxygen or nitrogen to give the desired ratios.
While such sterilizing processes involving oxidizing gases effectively kill organisms, including spores, without damage to the articles being sterilized and with no toxic residues or emissions presenting environmental safety hazards, monitoring the process has posed a challenge due to the reduced resistance of various commonly used biological indicator organisms to such processes. Further, Bacillus stearothermophilus, which is recognized as an organism useful in biological indicators for steam sterilizations, has been found to have a curvilinear response curve to oxidizing gas sterilization processes. This is disadvantageous because inactivation of an indicator organism must occur in a predictable manner. Thus, the number of viable indicator organisms in a biological indicator should decrease exponentially with exposure time to allow the sterilization process time to be adjusted with an appropriate safety margin. Under the right conditions, sterilization can approximate first order kinetics, and thus allow sterilization cycle times to be readily determined. Thus, better bacteriological indicators, particularly those suitable for gaseous oxidizing sterilants and sterilizing processes, would be useful.